This invention relates generally to electrical contact assemblies and more particularly to such assemblies used with test equipment for testing printed circuit boards and other electrical apparatus.
It is conventional to use spring biased contact assemblies, or probes, in testing printed circuit boards and other electrical apparatus for electrical continuity and the like. Typically, contact probes, generally comprising a center signal contact and an outer shield or ground contact, are mounted in a support and brought into relative movement with a device to be tested, such as a circuit board. Upon engagement with the test sites, relative movement is continued for a selected distance, e.g., 0.090 inch is a conventional distance used in the industry, to ensure optimum contact engagement. Tests are performed by sending electrical signals through the test sites to check for electrical continuity and the like.
In carrying out testing using such contact assemblies, it is important to minimize signal mis-timing, delay and degradation. This is especially true when the contact assemblies are used with high speed apparatus where a delay in digital signal propagation can result in false readings.
Such contact assemblies have moving parts and inherently have a degree of mismatch within the manufacturing tolerances and materials employed. Any mismatches which do occur are directly related to the length of the contact assemblies so that the longer the assembly, the greater the mismatch.
It is known to use double ended coaxial contacts as shown in U.S. Pat. No. 3,416,125 to Theve; U.S. Pat. No. 5,175,493 to Langgard; U.S. Pat. No. 5,936,421 to Stowers et al. and U.S. Pat. No. 6,053,777 to Boyle. However, each of the above contact assemblies has one or more limitations. In the Theve and Stowers et al. patents a common spring is used to provide contact force for the inner contacts and another common spring is used to provide contact force for the outer contacts with no means provided for independent selection of the force for each contact. In the Langgard patent the double ended probe assembly includes two probes in back-to-back relationship but separated from one another in a barrel requiring conductive epoxy or the like for filling in the void between the probes to maintain conductivity thereby introducing potential yield problems as well as possible adverse affects on signal integrity. In the patent to Boyle, a contact system is attached to opposite ends of a coaxial cable, as shown in FIG. 8 of the patent. Such a system typically uses 18 to 24 inches of coaxial cable. All cables used for a test set up must be of the same length to maintain signal integrity. It would be desirable, especially for ever higher speed applications, to have a more compact contact assembly available and one with improved flexibility and independence for providing particular characteristics of the several contacts of the assembly.
It is an object of the present invention to provide a double-ended contact assembly particularly useful for high frequency signal testing which is more compact than prior art devices yet one which provides improved impedance matching. Another object of the invention is the provision of a double ended coaxial contact assembly which is more conducive to adaptation to various customer requirements including independent choice of contact tip styles and spring forces for the ground plungers as well as the center probes.
Briefly, in accordance with the invention, a center probe rod is lockingly received in a generally cylindrical center dielectric spacer bushing. Each end of the center probe rod is formed with a bias surface inclined relative to the longitudinal axis of the center probe rod for engagement with a biasing ball of a respective center probe assembly received at each end of the center probe rod. The center spacer bushing and center probe rod subassembly is received in a ground shield sleeve with the longitudinal axial position of the subassembly suitably fixed relative to the ground sleeve as by detents formed in the ground sleeve in alignment with an annular groove formed in the outer periphery of the center spacer bushing.
Selected ground contact tip configurations are formed at opposite ends of the ground sleeve. Each end of the center probe rod is telescopically received for limited relative sliding movement in one end of a respective center probe barrel mounting a center probe contact having a selected contact tip configuration at the other end. A biasing spring is disposed in each center probe barrel between the center probe contact and the biasing ball to bias the center probe barrel outwardly limited by detents of the center probe barrel received in a reduced diameter portion of the center probe rod. The center probe barrel is also received in a respective dielectric outer spacer bushing to maintain coaxial relationship with the ground sleeve. In one embodiment the center probe rod is formed with an axially extending bore in which posts extending from the center probe contacts are slidingly received in opposite ends of the bore and with the center probe barrels biased outwardly by a respective coil spring received on the center probe rod between the center spacer bushing and the respective center probe barrel.
In other embodiments, the ground sleeve is provided with a tubular ground plunger contact slidingly received in each end of the ground sleeve and separate coil springs are disposed between each ground plunger contact and the center spacer bushing for biasing the ground plunger contacts outwardly. In one preferred embodiment, the center spacer bushing is provided with a sleeve which preferably extends axially to a position where the inboard end of the ground plunger contact overlaps the distal free end of the sleeve when the ground plunger contacts are in the depressed or actuated position. In yet another preferred embodiment, one end of the double-ended contact assembly is formed with an MMCX type coaxial connector plug.
Contact assemblies made in accordance with the invention are shorter in length than prior art devices, e.g., less than 1 inch in length, which allows for a more compact system. Each end of the assembly operates independently of the other end and is common only by the center probe rod and center spacer bushing subassembly fastened to the ground sleeve. This allows for the same center probe rod and center spacer bushing subassembly to be used with different tip styles and spring forces. Also, when used with movable ground plungers, different spring forces can be used on the two ends of the contact assembly.